The obesity epidemic is a global challenge that requires evidence-based intervention to slow the emergence of weight-related illness in children and adolescents. Thus, effective treatments are urgently needed. This study aimed to analyze whether there is relationship between the initial stage of readiness for behavior change (SRBC) and the results obtained through participation in a multidisciplinary program of obesity treatment (MPOT). The study included 113 overweight or obese children and adolescents who participated in an intervention for 16 weeks, in which the categorical variable was the SRBC. The dependent variables included: anthropometric measures of body composition, hemodynamic parameters (e.g., blood pressure values), and health-related physical fitness. Although stages 1 and 2 had greater improvements in flexibility than those in stage 5, and stage 1 had greater percentage changes in abdominal strength than those in stage 5, children and adolescents in the highest stage had greater percentage changes for anthropometric, hemodynamic and cardiorespiratory fitness, suggesting an impact of the stage of change on the effects of a MPOT. This study suggests that SRBC plays a role in obesity treatment in Brazilian children and adolescents and warrants consideration when enrolling patients to intervention.